Another model to consider: ATH-AD2000. Or what about some 'stats?
To get a feel for the HD650, my EQ for the (600 ohm) HD540 and Shibatch Super Equalizer 0.03 for Winamp should work decently. It would be the darkest-sounding of the models suggested (can handle loads of bass, too). Try one on to see whether it would be comfortable for you in the long run, the clamping should be less of an issue for a HD540 user (the deeper earcups may well be appreciated) but the cans are somewhat top-heavy and might press on your head more than you'd like.
K701s reportedly are good with small-scale classical, but for large-scale orchestral music the classic K501 might be better. (DT880s are also known to work well under these circumstances.) No clue 'bout AD2000 and SA5000.
The main question would be what you'd like to see improved. The HD540 (I assume the older 600 ohm version sounds pretty much the same as the "Gold") mainly is a good bit brighter than neutral and has rather shallow earcups, a good bit of clamping force but little weight on top (it also is rather light in general, more so than e.g. a HD580). I wouldn't mind a "Gold" myself, the looks certainly are neat (along with the nice wooden box, the HD650 only gets corrugated cardboard), but admittedly buying cans just for looks is a bit silly. (You actually have more flexibility with the plain 600 ohm HD540 - replacement earpads are cheaper and more comfortable, and one is less inclined to keep the standard steel cable instead of swapping it for a less microphonic OFC job when compared to the "Gold" one that was terminated in a massive gold-plated plug.)
